Lithium secondary batteries are small, lightweight, high capacity batteries which have been widely used as a power source for mobile devices since 1991. Recently, with rapid advancements in the electronics, communications, and computer industries, camcorders, mobile phones, notebook PCs, and the like, have appeared and continue to advance at an incredible pace. The demand for lithium secondary batteries as a power source for driving such mobile electronic communication devices continues to increase.
Lithium secondary batteries are limited in that lifetime rapidly decreases with repeated charging and discharging. In particular, this limitation is more severe at high temperatures. This is because electrolyte breaks down or active material degrades due to effects such as moisture inside of the battery, etc., or because of phenomena which occurs as the internal resistance of the battery increases.
Examples of lithium secondary battery positive electrode active materials which are being researched and developed accordingly include lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and lithium nickel cobalt manganese oxide (Li(NixCoyMnz)O2). However, in the case of LiNiO2, not only is synthesis difficult, but there is a limitation in thermal stability, and thus commercialization is difficult. In the case of LiMn2O4, although there has been some commercialization of low-priced products, lifetime properties are poor due to structural distortion caused by Mn3+ (Jahn-Teller distortion). In addition, although LiFePO4 has low cost and good stability and is thus being extensively researched for use in hybrid electric vehicles (HEV), application in other areas is currently limited due to a low conductivity.
Consequently, Li(NixCoyMnz)O2 has recently been the most prominent among positive electrode active materials which are being considered as a replacement for LiCoO2. Such material is more inexpensive than LiCoO2 and has advantages of high-capacity and of being able to be used at high voltage, but has disadvantages of poor rate capability and poor lifetime properties at high temperature. Extensive research, such as on a method for coating surfaces of electrode active material with metal oxide coating layers, has been carried out to overcome such limitations.
For example, Korean Patent No. 10-277796 discloses a technique for coating metal oxide by coating metals such as magnesium (Mg), aluminum (Al), cobalt (Co), potassium (K), sodium (Na), calcium (Ca), etc., on the surface of positive electrode active material and then performing heat treatment in an oxidizing atmosphere.
However, there still remain difficulties in overcoming the limitations of reaction between electrolyte and positive electrode active material, and structural transitions on the surface of positive electrode active materials due to the existence of impurities on the surface of positive electrode active materials. Consequently, there is a demand for a positive electrode active material which is capable of minimizing reduction in capacity or output and improving lifetime properties in secondary batteries by reducing addition reactions of electrolyte and active material during charging and discharging, and reducing the internal resistance of the battery.